1. Field of the Invention
The present invention relates to control the difference of phases of a plurality of signal sources and to control changes of frequencies thereof.
2. Description of the Related Art
When an impedance measurement instrument or the like is used, it may be necessary to set it so that signals that a plurality of signal sources generate have different phases. Recently, as such signal sources, a Direct Digital Synthesizer (abbreviated as a DDS) has been widely used. A DDS has a table of phases and signal output values and a Digital-Analog converter (hereinafter referred to as a D/A converter). The DDS can output a signal of a sine wave.
When the phase of the DDS is controlled, the phase shifting is affected by the bit resolution of a register that adjusts the phase offset. Thus, it was difficult to finely control the phase of the DDS. When an impedance is measured, it is necessary to accurately measure the gain of an amplifier of a voltage measurement section. To do that, it is necessary to improve the phase resolution.
To control to change the phase of the DDS that is operating, since a Band Pass Filter (hereinafter abbreviated as a BPF) is disposed at an output of the DDS, a transient takes place in the waveform that is output from the DDS. Thus, it takes a time until the waveform becomes stable.
In addition, when an impedance is measured, it is necessary to accurately measure the gain of the amplifier of the voltage measurement section as disclosed in a related art reference for example Japanese Patent Application Unexamined Publication No. 2004-317345.